1. Field of the Invention
The present invention relates generally to a cone-beam computed tomography system and, more particularly, to a cone-beam computed tomography system that employs an amorphous silicon flat-panel imager for use in radiotherapy applications where the images of the patient are acquired with the patient in the treatment position on the treatment table.
2. Discussion of the Related Art
Radiotherapy involves delivering a prescribed tumorcidal radiation dose to a specific geometrically defined target or target volume. Typically, this treatment is delivered to a patient in one or more therapy sessions (termed fractions). It is not uncommon for a treatment schedule to involve twenty to forty fractions, with five fractions delivered per week. While radiotherapy has proven successful in managing various types and stages of cancer, the potential exists for increased tumor control through increased dose. Unfortunately, delivery of increased dose is limited by the presence of adjacent normal structures and the precision of beam delivery. In some sites, the diseased target is directly adjacent to radiosensitive normal structures. For example, in the treatment of prostate cancer, the prostate and rectum are directly adjacent. In this situation, the prostate is the targeted volume and the maximum deliverable dose is limited by the wall of the rectum.
In order to reduce the dosage encountered by radiosensitive normal structures, the location of the target volume relative to the radiation therapy source must be known precisely in each treatment session in order to accurately deliver a tumorcidal dose while minimizing complications in normal tissues. Traditionally, a radiation therapy treatment plan is formed based on the location and orientation of the lesion and surrounding structures in an initial computerized tomography or magnetic resonance image. However, the location and orientation of the lesion may vary during the course of treatment from that used to form the radiation therapy treatment plan. For example, in each treatment session, systematic and/or random variations in patient setup (termed interfraction setup errors) and in the location of the lesion relative to surrounding anatomy (termed interfraction organ motion errors) can each change the location and orientation of the lesion at the time of treatment compared to that assumed in the radiation therapy treatment plan. Furthermore, the location and orientation of the lesion can vary during a single treatment session (resulting in intrafraction errors) due to normal biological processes, such as breathing, peristalsis, etc. In the case of radiation treatment of a patient's prostate, it is necessary to irradiate a volume that is enlarged by a margin to guarantee that the prostate always receives a prescribed dose due to uncertainties in patient positioning and daily movement of the prostate within the patient. Significant dose escalation may be possible if these uncertainties could be reduced from current levels (.about.10 mm) to 2-3 mm.
Applying large margins necessarily increases the volume of normal tissue that is irradiated, thereby limiting the maximum dose that can be delivered to the lesion without resulting in complication in normal structures. There is strong reason to believe that increasing the dose delivered to the lesion can result in more efficacious treatment. However, it is often the case that the maximum dose that can be safely delivered to the target volume is limited by the associated dose to surrounding normal structures incurred through the use of margins. Therefore, if one's knowledge of the location and orientation of the lesion at the time of treatment can be increased, then margins can be reduced, and the dose to the target volume can be increased without increasing the risk of complication in normal tissues.
A number of techniques have been developed to reduce uncertainty associated with systematic and/or random variations in lesion location resulting from interfraction and intrafraction errors. These include patient immobilization techniques (e.g., masks, body casts, bite blocks, etc.), off-line review processes (e.g., weekly port films, population-based or individual-based statistical approaches, repeat computerized tomography scans, etc.), and on-line correction strategies (e.g., pre-ports, MV or kV radiographic or fluoroscopic monitoring, video monitoring, etc.).
It is believed that the optimum methodology for reducing uncertainties associated with systematic and/or random variations in lesion location can only be achieved through using an on-line correction strategy that involves employing both on-line imaging and guidance system capable of detecting the target volume, such as the prostate, and surrounding structures with high spatial accuracy.
An on-line imaging system providing suitable guidance has several requirements if it is to be applied in radiotherapy of this type. These requirements include contrast sensitivity sufficient to discern soft-tissue; high spatial resolution and low geometric distortion for precise localization of soft-tissue boundaries; operation within the environment of a radiation treatment machine; large field-of-view (FOV) capable of imaging patients up to 40 cm in diameter; rapid image acquisition (within a few minutes); negligible harm to the patient from the imaging procedure (e.g., dose much less than the treatment dose); and compatibility with integration into an external beam radiotherapy treatment machine.
Several examples of known on-line imaging systems are described below. For example, strategies employing x-ray projections of the patient (e.g., film, electronic portal imaging devices, kV radiography/fluoroscopy, etc.) typically show only the location of bony anatomy and not soft-tissue structures. Hence, the location of a soft-tissue target volume must be inferred from the location of bony landmarks. This obvious shortcoming can be alleviated by implanting radio-opaque markers on the lesion; however, this technique is invasive and is not applicable to all treatment sites. Tomographic imaging modalities (e.g., computerized tomography, magnetic resonance, and ultrasound), on the other hand, can provide information regarding the location of soft-tissue target volumes. Acquiring computerized tomography images at the time of treatment is possible, for example, by incorporating a computerized tomography scanner into the radiation therapy environment (e.g., with the treatment table translated between the computerized tomography scanner gantry and the radiation therapy gantry along rails) or by modifying the treatment machine to allow computerized tomography scanning. The former approach is a fairly expensive solution, requiring the installation of a dedicated computerized tomography scanner in the treatment room. The latter approach is possible, for example, by modifying a computer tomography scanner gantry to include mechanisms for radiation treatment delivery, as in systems for tomotherapy. Finally, soft-tissue visualization of the target volume can in some instances be accomplished by means of an ultrasound imaging system attached in a well-defined geometry to the radiation therapy machine. Although this approach is not applicable to all treatment sites, it is fairly cost-effective and has been used to illustrate the benefit of on-line therapy guidance.
As illustrated in FIGS. 1(a)-(c), a typical radiation therapy system 100 incorporates a 4-25 MV medical linear accelerator 102, a collimator 104 for collimating and shaping the radiation field 106 that is directed onto a patient 108 who is supported on a treatment table 110 in a given treatment position. Treatment involves irradiation of a lesion 112 located within a target volume with a radiation beam 114 directed at the lesion from one or more angles about the patient 108. An imaging device 116 may be employed to image the radiation field 118 transmitted through the patient 108 during treatment. The imaging device 116 for imaging the radiation field 118 can be used to verify patient setup prior to treatment and/or to record images of the actual radiation fields delivered during treatment. Typically, such images suffer from poor contrast resolution and provide, at most, visualization of bony landmarks relative to the field edges.
Another example of a known on-line imaging system used for reducing uncertainties associated with systematic and/or random variations in lesion location is an X-ray cone-beam computerized tomography system. Mechanical operation of a cone beam computerized tomography system is similar to that of a conventional computerized tomography system, with the exception that an entire volumetric image is acquired through a single rotation of the source and detector. This is made possible by the use of a two-dimensional (2-D) detector, as opposed to the 1-D detectors used in conventional computerized tomography. There are constraints associated with image reconstruction under a cone-beam geometry. However, these constraints can typically be addressed through innovative source and detector trajectories that are well known to one of ordinary skill in the art.
As mentioned above, a cone beam computerized tomography system reconstructs three-dimensional (3-D) images from a plurality of two-dimensional (2-D) projection images acquired at various angles about the subject. The method by which the 3-D image is reconstructed from the 2-D projections is distinct from the method employed in conventional computerized tomography systems. In conventional computerized tomography systems, one or more 2-D slices are reconstructed from one-dimensional (1-D) projections of the patient, and these slices may be “stacked” to form a 3-D image of the patient. In cone beam computerized tomography, a fully 3-D image is reconstructed from a plurality of 2-D projections. Cone beam computerized tomography offers a number of advantageous characteristics, including: formation of a 3-D image of the patient from a single rotation about the patient (whereas conventional computerized tomography typically requires a rotation for each slice); spatial resolution that is largely isotropic (whereas in conventional computerized tomography the spatial resolution in the longitudinal direction is typically limited by slice thickness); and considerable flexibility in the imaging geometry. Such technology has been employed in applications such as micro-computerized tomography, for example, using a kV x-ray tube and an x-ray image intensifier tube to acquire 2-D projections as the object to be imaged is rotated, e.g., through 180.degree. or 360.degree. Furthermore, cone beam computerized tomography has been used successfully in medical applications such as computerized tomography angiography, using a kV x-ray tube and an x-ray image intensifier tube mounted on a rotating C-arm.
The development of a kV cone-beam computerized tomography imaging system for on-line tomographic guidance has been reported. The system consists of a kV x-ray tube and a radiographic detector mounted on the gantry of a medical linear accelerator. The imaging detector is based on a low-noise charge-coupled device (CCD) optically coupled to a phosphor screen. The poor optical coupling efficiency (−10.sup.−4) between the phosphor and the CCD significantly reduces the detective quantum efficiency (DQE) of the system. While this system is capable of producing cone beam computerized tomography images of sufficient quality to visualize soft tissues relevant to radiotherapy of the prostate, the low DQE requires imaging doses that are a factor of 3-4 times larger than would be required for a system with an efficient coupling (e.g. −50% or better) between the screen and detector.
Another example of a known auxiliary cone beam computerized tomography imaging system is shown in FIG. 2. The auxiliary cone beam computerized tomography imaging system 200 replaces the CCD-based imager of FIGS. 1(a)-(c) with a flat-panel imager. In particular, the imaging system 200 consists of a kilovoltage x-ray tube 202 and a flat panel imager 204 having an array of amorphous silicon detectors that are incorporated into the geometry of a radiation therapy delivery system 206 that includes an MV x-ray source 208. A second flat panel imager 210 may optionally be used in the radiation therapy delivery system 206. Such an imaging system 200 could provide projection radiographs and/or continuous fluoroscopy of the lesion 212 within the target volume as the patient 214 lies on the treatment table 216 in the treatment position. If the geometry of the imaging system 200 relative to the system 206 is known, then the resulting kV projection images could be used to modify patient setup and improve somewhat the precision of radiation treatment. However, such a system 200 still would not likely provide adequate visualization of soft-tissue structures and hence be limited in the degree to which it could reduce errors resulting from organ motion.
Accordingly, it is an object of the present invention to generate KV projection images in a cone beam computerized tomography system that provide adequate visualization of soft-tissue structures so as to reduce errors in radiation treatment resulting from organ motion.